Vacuum lines of the above kind are used, among other things, to convey cryogenic liquids, the vacuum prevailing between the inner transport pipe and the surrounding outer pipe forming an effective insulation. Such vacuum line arrangements may, of course, also be used beneficially for the transportation of hot media.
Difficulties are experienced in the case of some applications in which two sections of such a vacuum line are to be coupled together in a manner which will enable the sections to be rotated relative to each other, for instance when the vacuum line is to be used for the transportation of a coolant, for example liquid nitrogen, to movable equipment, these difficulties being particularly applicant when the application concerned requires frequent rotation between the line sections. One problem in this respect resides in the fact that the requisite elastic sealing rings are unable to withstand the low or high temperatures concerned. Normally, a sealing ring made of some suitable material, for instance a rubber composition, cannot be subjected to temperatures below 40.degree. C. and remain effective. This temperature shall be compared with the temperature of liquid nitrogen, that is -196.degree. C., which one is often desirous of using for cooling (chilling) purposes among others.